The United States domestic spy agency monitored the phone conversations of 35 world leaders using numbers supplied by a US government official, according to a report.

Citing classified documents leaked by fugitive whistleblower Edward Snowden, Britain's The Guardian added that staff in the White House, State Department and the Pentagon were urged to share the contact details of foreign politicians with the National Security Agency.

"In one recent case, a US official provided NSA with 200 phone numbers to 35 world leaders," reads an excerpt from a confidential memo dated October 2006 which was quoted by The Guardian.

The identities of the politicians in question were not revealed.

The report suggests that the bugging of world leaders could be more widespread than originally thought, with the issue set to overshadow a European Union summit in Brussels.

The White House has not denied the bugging, saying only that it would not happen in future.

No immediate comment on the report was available from the NSA.

Merkel calls US spying a breach of trust

Meanwhile, the German chancellor Angela Merkel has publicly expressed her anger at reports that US intelligence agencies have monitored her mobile phone.

Arriving at the Brussels summit, Ms Merkel told reporters that spying between friends was not at all acceptable.

"This is not just about me, but it's about all German citizens. Partnerships are based on trust and we now have to rebuild trust," she said.

"I repeat that spying among friends is not at all acceptable against anyone, and that goes for every citizen in Germany."

She confirmed that she had made the same point to the US president Barack Obama in a telephone call after reports emerged that her phone calls or texts had been monitored.

Her stern words follow an announcement by the German government on Wednesday that it had seen evidence suggesting the chancellor's mobile was "monitored" by the NSA.

Germany's foreign minister has summoned the US ambassador to Berlin to discuss the issue, an event diplomats said had rarely happened in the past 60 years.

The White House has so far only confirmed that the German chancellor's phone is not and will not be monitored, without ruling out that that happened in the past.

Angela Merkel is understood to have met the French president Francois Hollande on the sidelines of the summit to discuss the issue.

EU president Herman Van Rompuy announced later on Thursday (local time) that France and Germany want their own talks with the US by the end of the year to try and settle the spying row.

The affair dredges up memories of eavesdropping by the Stasi secret police in the former East Germany, where Ms Merkel grew up, and is an emotive topic for many Germans.

NSA spying reports multiplying

Following the unceasing flow of leaks by former US data analyst Edward Snowden, which revealed the reach of the NSA's data-collection programmes, Washington finds itself at odds with a host of important allies, from Brazil to Saudi Arabia.

Germany's frustration follows outrage in France after Le Monde newspaper reported the NSA had collected tens of thousands of French phone records between December 2012 and January 2013.

An Italian news magazine reported on Thursday that the NSA had monitored sensitive Italian telecommunications.

The revelations could have an impact on major legislative and trade initiatives between the United States and the European Union, with some German lawmakers saying negotiations over an EU-US free-trade agreement should be suspended.

Ms Merkel, who has previously discussed a "no spying" agreement with the United States, hinted that data-sharing deals with Washington may need to be relooked at, a potentially damaging blow for US efforts to collect counter-terrorism information.ter

"To this end, we need to ask what we need, which data security agreements we need, what transparency we need between the United States of America and Europe," Merkel said.

"We are allies facing challenges together. But such an alliance can only be built on the basis of trust."

"The agencies know perfectly well that every country, even when they cooperate on anti-terrorism, spies on its allies. The Americans spy on us on the commercial and industrial level like we spy on them, because it's in the national interest to defend our businesses. No one is fooled."